I found a solution here https://wiki.squid-cache.org/SquidFaq/ConfiguringSquid#How_do_I_configure_Squid_forward_all_requests_to_another_proxy.3F
it was necessary to put those three lines in the /etc/squid/squid.conf file:
cache_peer proxy.company.de parent 3128 3130
Ok, it looks like an incorrect configuration setting of the squid.conf file.
For anyone in the future would come here...
I setup the access control properties by using Webmin (Servers > Squid proxy server > Proxy restrictions): I had allowed the snmp_community called public (which is the one I use) but this created a http_access allow rule in the ...